An expat New Zealander living in Sydney has spoken of her fears after a metal bar burst through the floor of a train carriage and narrowly missed her head as she was travelling from to the city's CBD during rush hour yesterday.

Kiriana Buffett, who is originally from Whakatane, instinctively took evasive action when a length of metal attached to a walkway between Edgecliff and Kings Cross stations pierced the floor and curled towards her about 8pm (NZT).

"I just remember seeing it punch through the floor and then towards me - I don't know what happened next," said Buffett, who sat in shock for 20 minutes after the incident.

"Someone said it slowed down but I didn't see it go slow at any stage. I'm pretty lucky to be alive actually."

The train derailed shortly after leaving Edgecliff station, none of the estimated 700 passengers aboard the eight carriage service was injured but the busy commuter line linking the CBD with Sydney's eastern suburbs was severely disrupted.

Buffett said passengers detected a burning smell from the moment they boarded the train.

"We all knew something was wrong from the beginning," she said.

"When we started moving there was really loud knocking all the way, and when we stopped at Edgecliff it was like a mad explosion. I saw it come through the floor, and when it came through the floor it just went over my head."

Fellow passenger Peter Pulu said: "It was just like a monster coming up from the floor. It was not that fast. It went up and hit the roof."

"It was terrifying," said Nicola Walsh. "There was a major smell of burning rubber."

The chief executive of Sydney Trains, Howard Collins, said the bar that broke through the carriage was a piece of "metal channelling" attached to a concrete walkway near Edgecliff Station.

"It was a corner piece of metal, literally that that covers the edge of the concrete," Collins said, adding: "It is too early to say this has anything to do with the actual cause of it."

He said the derailment may have occurred through "either something connected to the train which has damaged the infrastructure of the track, or it was something on the track."

Investigators and Sydney Trains repair crews worked overnight to check the line and clear the damage.

The Eastern Suburbs line was closed between Bondi Junction and Martin Place for several hours after the incident, normal service expected to resume today according to a spokeswoman for the Transport Management Centre.

A Fairfax Media producer who was in the carriage, Annie Dang, said there was loud screeching noises before the train shuddered to a halt. Dang did not see the bar emerge through the base of the carriage, but said passengers nearby were still in shock.

"All I heard was a loud screeching sound and then the train started rattling really loudly and then people started screaming and running," she said.

Dang said she spoke to a woman standing right next to the bar when it emerged. "It probably would have missed her by 30 centimetres," she said. "It came right close to her face."

The train was between Edgecliff Station and Kings Cross, heading to the city, when it derailed.

Photos taken by a train passenger show the large piece of metal piercing the floor of the train carriage.